In digital image systems (such as digital TV systems), in order to boost video quality, images are often enhanced in many different aspects, such as image details, image contrast, etc., before being displayed. For a digital image, the quality of edges in the image (i.e., image edges) are of great importance to the overall image quality. Therefore, during the enhancement processes, each image edge should be well preserved. In other words, image enhancement should not sacrifice image edge quality. Otherwise, the overall visual quality of the image may be degraded even though the image is enhanced in certain aspects.
An instance of such visual quality degradation is a zigzagged edge artifact due to conventional image detail enhancement. As the goal of image detail enhancement is to improve the image sharpness, high frequency image components that contain image details are extracted, enhanced and then added back to the original image. As a result, image details in the processed image become more obvious than those in the original image. However, some artifacts can also be introduced into image edges because of the enhancement.
An example of the zigzagged edge artifact is described in conjunction with FIGS. 1A-B. FIG. 1A shows an original image edge 10, wherein each small square block 11 in the edge 10 represents one image pixel. The edge direction has a low angle relative to the horizontal direction. Along the horizontal direction across the edge, there is a long luminance transition range 12 from dark area to bright area or vice versa, as indicated in FIG. 1A. The luminance transition range 12 refers to the length of the luminance transitioning area of an edge either along the horizontal direction or along the vertical direction across the edge. The boundary of the edge shown in FIG. 1A looks generally smooth even though the edge has a limited image resolution.
A conventional image detail enhancement process is applied to the image edge 10 of FIG. 1A to generate the enhanced image 14 in FIG. 1B. Because in an image detail enhancement process high frequency components are boosted, the luminance transition range 12 may become shorter (or sharper). As can be seen in FIG. 1B, the luminance transition range 12 along the horizontal direction has become much shorter relative to that in FIG. 1A. As a consequence, the edge boundary now includes zigzagged edge artifacts. The more the image is enhanced in the detail enhancement process, the more obvious this kind of artifact would be. As a result, even though the image in FIG. 1B is enhanced, the quality of the image looks poor due to the degradation of edge quality.
The problem shown in FIG. 1B exists for most slant image edges. A slant image edge refers to an image edge whose direction is not exactly vertical or horizontal. Only when an image edge has exactly vertical or horizontal or precisely ±45°(i.e. +45° or −45°) direction, it is immune to the problem shown in FIG. 1B. Otherwise, a slant image edge can develop zigzagged edge artifacts if it is enhanced substantially. The closer the slant image-edge direction is to horizontal or vertical direction, the easier it is for a slant image edge to develop zigzagged edge artifact when enhanced substantially and the more obvious the artifact would be.
Therefore, there is a need for a method of detecting the location and the luminance transition range of slant image edges so that pixels belonging to a slant edge can be processed separately or differently from the other image areas to avoid possible edge artifacts.